The invention relates to a method for processing locating pulses supplied by a gamma camera and to a gamma camera applying this method. It relates to scintillation or gamma cameras of the ANGER type, its principles of operation and embodiments being disclosed in U.S. Pat. No. 3,011,057. These gamma cameras are intended to detect and provide a visual display of photons emitted by radioactive bodies.
Gamma cameras are utilised in nuclear medicine in order to establish a visual display of the distribution within an organ of molecules tagged by a radioactive isotope which had been injected into a patient. A gamma camera commonly comprises a collimator for focussing the gamma photons emitted by the patient, a scintillator crystal for converting the gamma photons into luminous photons or scintillations, and an array of photomultiplier tubes which in each case convert the scintillations into electrical pulses referred to as electrical tube contributions. It also comprises electronic circuits for deriving from the electrical contributions provided by the photomultiplier tubes signals giving the X and Y coordinates of the location at which the scintillation had occurred, as well as a signal Z for validation when the scintillation energy W lies within a predetermined energy range.
This detection chain is followed by a visual display system commonly comprising a cathode-ray oscilloscope controlled by the signals giving the X and Y coordinates and the signal Z in order to produce a visual display in the form of a bright dot on the screen of the point of impact of the gamma photon on the crystal. The visual display system may possibly incorporate a photographic device to establish an image of the organ examined by integrating a very large number of bright dots generated on the cathode-ray tube screen. It may also comprise a device for digital processing of the images.
Amongst other qualities, a gamma camera should have a satisfactory spatial resolution, that is to say the capacity of distinguishing closely spaced small radioactive sources, a satisfactory response in respect of counting rate, that is to say the capacity to process a large number of events per unit of time, and an image quality unaffected by the energy of the isotope in question.
The spatial resolution depends on the accuracy of calculation of the X and Y coordinates. The quality of the calculation of these coordinates depends substantially on the physical principles governing the operation of the different parts of the gamma camera. Thus, the interaction between a gamma photon and the crystal causes a luminous scintillation of which the intensity decreases exponentially with time. The time constant of this reduction is characteristic of the scintillator crystal utilised. For a thallium-activated sodium iodide crystal NaI(Tl), it is of the order of 250 nanoseconds. This scintillation is detected simultaneously by several photomultiplier tubes. The luminous photons forming this scintillation release photoelectrons from the photocathodes of the photomultiplier tubes. The number of photoelectrons released follows POISSON's statistical law for a given scintillation. This means that the electrical contribution of a photomultiplier tube receiving a scintillation has an amplitude of a value which follows a Poisson statistical distribution and of which the mean value is a function of the energy of the incident luminous photons. Moreover, at constant energy this electrical contribution is a substantially Gaussian function of the distance separating the centre of this photomultiplier tube from the location at which the scintillation had occurred. If the scintillation occurs in line with the centre of this tube, the electrical contribution is a maximum; the electrical contribution decreases with increasing distance of the scintillation location from the centre of the tube. By way of example, if a scintillation occurs in line with a wall of the tube, its electrical contribution is reduced by approximately half as compared to the maximum electrical contribution.
A scintillation is detected at the same time by several photomultiplier tubes, commonly by 6 to 10 tubes. The determination of the location of this scintillation on the crystal, which itself illustrates the point of emission of the energising gamma photon, may be obtained by calculating the position of the barycentre of the electrical contributions provided by the assembly of the photomultiplier tubes energised by this scintillation. According to ANGER, this calculation is performed in a simple manner by injecting the electrical contributions through a set of matrices of resistors whose resistivity values are a function of the positions of the photomultiplier tubes to which they are connected. The positions of these tubes are located with respect to Cartesian reference axes of which the point of intersection is commonly situated at the centre of the network of tubes. The set of matrices commonly comprises five matrices: four matrices serving the purpose of location, and one matrix being utilised to measure the energy. There are as many resistors in each matrix as there are photomultiplier tubes in the network of tubes. Each of the resistors is connected on the one hand to the output of a different photomultiplier tube, and on the other hand to a common point forming the output of the matrix. In this manner, these resistors establish a balance of the electrical contributions of each of the photomultiplier tubes supplying these.
A problem which is difficult to resolve in respect of a given scintillation, is that of determination with optimum precision of the mean values of the amplitudes of each of the electrical contributions. It is known that these contributions may be integrated in time over a period of the order of the decay time constant of the scintillations of the scintillator crystal. The period of this integration typically amounts to the order of three times the time constant. The period of integration required is a direct consequence of the Poisson statistic. As a matter of fact, the typical difference or deviation of the fluctuation of the amplitude of the electrical contributions according to the Poisson statistic is inversely proportional to the square root of the number of photoelectrons released. Thus, the longer the integration, the greater will be the number of photoelectrons taken into account and the smaller the typical deviation and the more precisely will the mean value of this contribution be assessed with precision. As a matter of fact, the operation for calculating the location of the barycentre being a linear operation, it is more economical to perform this integration at the output of each of the matrices of resistors of the set of matrices. In effect, these matrices merely establish a weighting or balance of the contributions of each tube as a function of the location of the tube on the crystal. The electrical pulses supplied at the output of the matrices of the set of matrices of resistors are referred to as weighted or balanced pulses. It will be noted in passing that the period of integration is thus linked directly to the quality of spatial resolution of the gamma camera, but that this quality is obtained at the expense of the counting rate, that is to say, at the expense of the number of events per second taken into account.
This integrating operation does not operate without some difficulties. The main one of these consists in the presence of constant direct voltages which are superimposed over the balanced pulses supplied by the matrices and which upon being fed into integration falsify the value of the signal supplied by these to an extent in direct proportion with the period of integration. The origin of these direct voltages consists principally in the presence of variable gain amplifiers interposed between each resistor matrix and a corresponding integrator. These variable gain amplifiers are utilised to perform an amplitude matching of the balanced pulses to the operational dynamics of the utilised integrators, and as a result for selection of an energy range to be investigated. These direct voltages which should be eliminated may have other origins, in particular that resulting from an action referred to as clutter of scintillations. In a patent application filed on the same day, the applicants disclosed a device making it possible to eliminate these direct voltages from the useful signal.
The balanced or weighted pulses denoted by x.sup.+, x.sup.-, y.sup.+ and y.sup.- are converted by the integrating operation after being received from the locating matrices, into so-called locating pulses generally denoted by X.sup.+, X.sup.-, Y.sup.+ and Y.sup.-. These locating pulses are fed into a calculator circuit which supplies the X and Y coordinate signals. The signals X and Y are proportional, respectively, to (X.sup.+ -X.sup.-) and to (Y.sup.+ -Y.sup.-) Now, the amplitudes of the locating pulses derive from the energy W of the scintillation which had caused these to be generated. It is known in the prior art to get rid of this energy by performing a standardisation of X and Y in the calculator circuit, in the form: ##EQU1## whilst yielding significant results, this method has particular shortcomings.
In effect, the energy W is obtained by integration as for the locating pulses X.sup.+,X.sup.-,Y.sup.+ and Y.sup.-, from a balanced pulse w supplied by a so-called power matrix of the set of matrices. All the balancing resistors are practically identical in the power matrix. They are adjustable at the manufacturers plant from one photomultiplier tube to another so that the power response is the same under each tube notwithstanding the position of this tube on the crystal. It follows that the energy W will be the same for scintillations of identical energy in each case, but occurring at random points on the scintillator. This statement should be qualified however by the fact that for each scintillation it is the mean value of the different W's and the variance of W which are the same since the photomultipliers are performing detecting operations in accordance with Poisson's statistic. On the contrary, in the case of the matrices serving the purpose of location, that is to say supplying balanced pulses x.sup.+,x.sup.-,y.sup.+ and y.sup.-, the resistances vary as a function of the position of the photomultiplier tubes on the crystal. For example, if x.sub.i denotes the abscissa of a tube with respect to the Cartesian reference axes, the resistance of the matrix supplying x.sup.+ have values equal to: ##EQU2## in which R.sub.o is a characteristic resistance selected as a function of the output impedance of the matrices, and in which D is the length of the median of the grid of tubes. The resistances of the matrix supplying x.sup.- have values equal to: ##EQU3## and similar values are allocated to the resistors of the matrices supplying y.sup.+ and y.sup.- as a function of the ordinate y.sub.i of the photomultiplier tube in question. These balancing resistors thus comply with hyperbolic functions. As a result, the variance of the locating pulses (or their degrees of freedom) X.sup.+,X.sup.-,Y.sup.+ and Y.sup.- depends on the point at which the scintillation had occurred, since the resistors of the locating matrices then perform precisely different balancing actions as a function of this point on the corresponding electrical contributions. Since the variance of W is constant, the standardisation of X or Y by W is consequently falsified.
Furthermore, it is known that the locating matrices may include non-linear elements in series with each of the resistors between the output of these resistors and the output of the matrix. The purpose of these non-linear elements is to eliminate excessively weak electrical contributions, that is to say having a level of the same order as that of the scintillation and detection noises. Non-linear elements of this kind are absent from the power matrix. As a result, by standardising X and Y as apparent from the foregoing, a linear function W of the scintillation energy standardises signals (X.sup.+ -X.sup.-) or (Y.sup.+ -Y.sup.-) which are not strictly proportional to this scintillation energy. Consequently, this is another cause of errors in calculating X and Y.
Another mode of standardisation consists in allowing for the sum of the locating pulses. In this case, the following expressions are applied: ##EQU4##
This solution is not satisfactory, as demonstrated by experimental results. As a matter of fact, for scintillations which all occur at one and the same ordinate (y.sup.i) the sum of the locating pulses (X.sup.+ +X.sup.-) is not constant and the values of Y resulting from the second formula are consequently different, whereas they are precisely those which should be equal since they apply to scintillations having the same ordinate.